It is known that by treating a cellulosic material in the presence of a catalytic amount of an N-oxyl compound and a chlorine based oxidizing agent, primary hydroxyl groups of cellulose can be oxidized into carboxyl groups and aldehyde groups (Non-patent Document 1).
The N-oxyl compound used as a catalyst in the above oxidized cellulose production technique is very expensive. Thus, it is desired to recover the N-oxyl compound from the effluent after completion of the reaction and then reuse the recovered N-oxyl compound. However, there has not been reported any method of recovering an N-oxyl compound from an effluent and reusing the N-oxyl compound thus recovered.